1. Field of the Invention
The present invention relates to a light-emitting gallium nitride-based compound semiconductor device and, more particularly, to a light-emitting compound semiconductor device having a double-heterostructure capable of emitting high-power visible light ranging from near-ultraviolet to red, as desired, by changing the composition of a compound semiconductor constituting an active layer (light-emitting layer).
2. Description of the Related Art
Gallium nitride-based compound semiconductors such as gallium nitride (GAN), gallium aluminum nitride (GaAlN), indium gallium nitride (InGaN), and indium aluminum gallium nitride (InAlGaN) have a direct band gap, and their band gaps change in the range of 1.95 eV to 6 eV. For this reason, these compound semiconductors are promising as materials for light-emitting devices such as a light-emitting diode and a laser diode.
For example, as a light-emitting device using a gallium nitride semiconductor, a blue light-emitting device in which a homojunction structure is formed on a substrate normally made of sapphire through an AlN buffer layer has been proposed. The homojunction structure includes a light-emitting layer formed of p-type impurity-doped GaN on an n-type GaN layer. As the p-type impurity doped in the light-emitting layer, magnesium or zinc is normally used. However, even when the p-type impurity is doped, the GaN crystal has a poor quality, and remains an i-type crystal having a high resistivity almost close to an insulator. That is, the conventional light-emitting device is substantially of a MIS structure. As a light-emitting device having the MIS structure, layered structures in which Si- and Zn-doped, i-type GaAlN layers (light-emitting layers) are formed on n-type CaAlN layers are disclosed in Jpn. Pat. Appln. KOKAI Publication Nos. 4-10665, 4-10666, and 4-10667.
However, in the light-emitting device having the MIS structure, both luminance and light-emitting output power are too low to be practical.
In addition, the light-emitting device of a homojunction is impractical because of the low power output by its nature. To obtain a practical light-emitting device having a large output power, it is required to realize a light-emitting device of a single-heterostructure, and more preferably, a double-heterostructure.
However, no light-emitting semiconductor devices of a double-heterostructure are known, in which the double-heterostructure is entirely formed of low-resistivity gallium nitride-based compound semiconductors, and at the same time, has a light-emitting layer consisting of low-resistivity, impurity-doped InGaN.
Jpn. Pat. Appln. KOKAI Publication Nos. 4-209577, 4-236477, and 4-236478 disclose a light-emitting device having a double-heterostructure in which an InGaN light-emitting layer is sandwiched between an n-type InGaAlN clad layer and a p-type InGaAlN clad layer. However, the light-emitting layer is not doped with an impurity, and it is not disclosed or explicitly suggested that an impurity is doped into the light-emitting layer. In addition, the p-type clad layer is a high-resistivity layer in fact. A similar structure is disclosed in Jpn. Pat. Appln. KOKAI Publication No. 64-17484.
Jpn. Pat. Appln. KOKAI Publication 4-213878 discloses a structure in which an undoped InGaAlN light-emitting layer is formed on an electrically conductive ZnO substrate, and a high-resistivity InGaN layer is formed thereon.
Jpn. Pat. Appln. KOKAI Publication No. 4-68579 discloses a double-heterostructure having a p-type GaInN clad layer formed on an oxygen-doped, n-type GaInN light-emitting layer. However, another clad layer consists of electrically conductive ZnO. The oxygen is doped in the light-emitting layer to be lattice-matched with the ZnO. The emission wavelength of the light-emitting device having this double-heterostructure is 365 to 406 nm.
All conventional light-emitting devices are unsatisfactory in both output power and luminance, and have no satisfactory luminosity.